GCSE Biology (Combined) Edexcel

Key Concepts: Cells

• The basic unit of all organisms, from bacteria to plants and humans, is the cell.
• There are two main types of cells: prokaryotic (typically bacteria) and eukaryotic (found in plants and animals).
• Prokaryotic cells are smaller and simpler, lacking a nucleus and most other organelles. Instead, their genetic material is in the cytoplasm.
• Eukaryotic cells, on the other hand, have a nucleus that houses the cell's DNA. They also contain specialised organelles, each with a specific function.
• Key organelles to understand are the nucleus (controls the cell's activities), mitochondria (where most energy is released in respiration), cytoplasm (where most of the cell's activities take place), cell membrane (controls what goes in and out of the cell), ribosomes (where proteins are made) and, in plant cells, chloroplasts (where photosynthesis happens) and a cell wall (made of cellulose, gives structure and support).
• Different types of cells have different forms, relating to their function. For example, nerve cells are long to carry signals over long distances, while root hair cells in plants are thin and elongated for maximum absorption of water and minerals.
• Cells divide in a process called cell division. In body cells this is mitosis, creating two identical daughter cells for growth and repair. In sex cells this is meiosis, creating four non-identical daughter cells for sexual reproduction.
• The cycle of cell growth and division is controlled by the cell cycle. Any errors in this process can lead to uncontrollable growth or cancer.
• Substances move in and out of cells via diffusion, osmosis, and active transport. Each of these processes involve different mechanisms and energy requirements.
• Photosynthesis, the process where green plants and algae convert carbon dioxide and water into glucose and oxygen using light energy, happens within chloroplasts.
• The rate of photosynthesis can be affected by light intensity, carbon dioxide concentration, and temperature.
• Respiration, where energy is released from the breakdown of glucose, happens within cells. It can be either aerobic (with oxygen) or anaerobic (without oxygen, also known as fermentation). Both forms of respiration have real-world applications and implications, for example in yeast fermentation for bread and beer production.